Communication technology has progressed significantly in the past few years. Today, much information is carried over optical fiber. Optical fibers are capable of transporting information at data rates currently exceeding billions of bits per second. Part of the technology that enables communication using optical fibers is the ability to direct signal light onto an optical fiber and to switch that light appropriately. It is also desirable to have the ability to selectively filter the signal light that is passing through the optical fiber.
Currently, fiber Bragg gratings (FBG) are typically used to provide wavelength-specific filtering of the signal light propagating through an optical fiber. An FBG is in a region of an optical fiber in which the refractive index of the core repetitively alternates between a high value and a low value along the length of the fiber. An FBG is fabricated by temporarily illuminating a photosensitive optical fiber with a light pattern generated by passing light from an ultraviolet (UV) laser through a phase mask. The resulting pattern of alternating illumination intensity establishes a region in the optical fiber in which the refractive index of the core alternates as described above. The region of alternating refractive index remains and acts as an FBG with a fixed optical filter characteristic. The FBG can be structured to produce various filter characteristics, such as a notch filter, a broadband filter, etc. Optical fibers incorporating one or more FBGs are typically used in fixed add/drop multiplexers, erbium doped fiber amplifier (EDFA) gain flatteners, dispersion compensators and fiber lasers, for example.
An FBG can be tuned by mechanically elongating the optical fiber by heating and/or mechanically stretching the fiber. Elongating the fiber changes the spatial frequency of the region of alternating refractive index and, hence, the optical filter characteristic provided by the FBG. Unfortunately, mechanically elongating the optical fiber can significantly reduce the long-term stability of the fiber. Moreover, mechanically elongating the optical fiber to change the spatial frequency of the region of alternating refractive index changes only the center wavelength of the filter characteristic. Mechanical elongation cannot be used to change the characteristics of the filter passband.
Therefore, there is a need for an optical waveguide having a core whose refractive index can be spatially reconfigured at will for use in the applications described above and in other applications. There is also a need for an optical waveguide having a core whose refractive index is polarization-independent.